refactor(locators): move findElementsOverrideHelper to a member of Protractor

Also fix a small change introduced in the last commit where the dialog was
never closed.
feat(locators): add the ability to add custom element locators with by.addLocator

Custom locators can now be added using by.addLocator(<name>, <script>), where
script is a self-contained snippet to be executed on the browser which returns
an array of elements. Closes #236.
chore(mergefix): Merge branch 'master' of https://github.com/angular/protractor

Conflicts:
	spec/basic/lib_spec.js